1. Field of the Invention
The present invention relates to an artificial kidney, and more particularly to an artificial kidney for treating people who are temporarily deprived of the use of their own kidneys following an accident or a surgical operation,
2. Description of the Related Art
Given the general state of weakness in which such patients are found, they cannot be subjected to the same intensive treatment as is given to patients suffering permanent kidney failure: i.e., a twice-weekly four-hour conventional hemodialysis or hemofiltration session. The rapid change in internal liquid balances that occurs in a traditional treatment approach places the cardiovascular system under intense stress that patients leaving the operating theater are generally not fit to withstand.
It is therefore the practice to purify the blood of such patients and to eliminate a portion of the water that accumulates in their tissues by using treatments that are not very intense but that are continuous, which treatments are easily tolerated by the body since there are no sudden changes, and are acceptable for people who are in no condition to move about.
Conventionally, the above-mentioned patients are subjected to two types of treatment: continuous hemofiltration and continuous hemodialysis.
Hemofiltration is based on removing from blood a portion of the impurity-containing plasma water therefrom by ultrafiltration through a semipermeable membrane. The transfer is driven by the pressure difference across the membrane. A substitution liquid may simultaneously be perfused into the patient, generally in smaller quantities than the filtrate that is removed, with the difference corresponding to the weight that it appears appropriate for the patient to loose. The ultrafiltration throughput, i.e. the efficiency of the hemofiltration treatment, is limited both by the characteristics of the filter (essentially the nature and the area of the membrane), and by the rate of flow of blood through the filter, which rate of flow is relatively low during continuous treatment (4 to 12 liters per hour as compared with 12 to 21 liters per hour during the treatment of patients with a permanent loss of kidney function).
During hemodialysis treatment, impurities in the blood are not entrained by convection in a flow of plasma water passing through a semipermeable membrane, as is the case in hemofiltration, but instead are diffused through a semipermeable membrane whose face that is not immersed in blood is irrigated by a flow of dialysis liquid that is free of the substances to be removed. Transfer is then driven by the concentration differences across the membrane.
For a given type of high permeability filter, for the same blood flow rate, and for an appropriate choice of dialysis liquid flow rate, it is possible to obtain much more effective purification of low molecular weight impurities with hemodialysis than it is with hemofiltration. In contrast, in addition to eliminating a portion of the water that accumulates in tissue, hemofiltration also makes it possible to eliminate impurities of high molecular weight that migrate little or not at all by diffusion.
From the above, it will be understood that to purify efficiently the blood of a patient suffering from kidney failure it is desirable to subject the patient to both treatment by hemofiltration and by hemodialysis.
European patent number 0 256 956 describes an artificial kidney enabling these treatments to be performed in alternation or simultaneously. That kidney includes an exchanger having two compartments that are separated by a semipermeable membrane. A first compartment is connected to a circuit for blood flow outside the body. The second compartment has an inlet that is connectable via a three-port valve to a first container of sterile dialysis liquid, and an outlet connected to a second container for collecting the waste dialysis liquid and the blood filtrate. The three-port valve also serves to connect the first container to the blood circuit downstream from the exchanger, with the sterile dialysis liquid being usable as an injectable substitution liquid. The sterile liquid may flow spontaneously or it may be driven by pumps, and the same applies to extracting the filtrate or the waste dialysis liquid.
With that artificial kidney, the method of treatment is selected manually by operating the three-port valve, on the prescription of the doctor.